A digital radio receiver to receive a satellite wave or ground wave so as to listen to digital radio broadcast has been developed and put in practical use in the United States of America. A digital radio receiver of this kind is generally mounted on a mobile object such as an automobile, and is designed to receive a radio wave having a frequency of about 2.3 GHz to listen to radio broadcast. For example, a Sirius satellite radio antenna apparatus normally serves to receive circular polarized radio waves from three orbiting satellites (synchronous type) and, in an insensitive zone, receive a radio wave from a ground linear polarization facility.
A digital radio receiver has an antenna apparatus for receiving a radio wave. This antenna apparatus is attached outside a mobile object. Also, this antenna apparatus is comprised of an antenna unit and an antenna case for covering this antenna unit. The antenna case is comprised of a dome-shaped top cover and a bottom plate. The antenna unit is comprised of an antenna element, a circuit board and a shield case. The antenna element comprised of, for example, a ceramic patch antenna, and receives a radio wave. The circuit board is provided with a signal processing circuit for performing various kinds of signal processing such as signal amplification upon a signal received by the antenna element. The shield case serves to shield the signal processing circuit from outside electric field and outside magnetic field.
The patch antenna is provided with an antenna electrode, an earthing conductor, a ceramic antenna board placed between the antenna electrode and the earthing conductor, and a feed pin that is electrically connected with the antenna electrode. A patch antenna that provides a top plate having a rectangular hole as a waveguide path on this antenna electrode is known (see, for example, patent literature 1). By providing a top plate having a rectangular hole, it is possible to increase the directional gain of the patch antenna at a high angle of elevation.